1. Field of the Invention
This invention relates to a method for producing an oxide superconductor and more particularly to a method for producing an A-Ce-Cu-O type oxide superconductor (wherein A is a lanthanide element or a rare earth element) or a Bi oxide superconductor.
2. Description of the Related Art
Recently, attempts to apply a metal or alloy superconductor to a device have been made. Nevertheless, since the critical temperature Tc is very low, such a device could not be used in liquid helium. On the other hand, an oxide superconductor has a high Tc and has the ability to be used in liquid nitrogen.
As the oxide superconductor a hole injection type oxide superconductor such as the yttrium (Y) or the bismuth (Bi) type and an electron injection type oxide superconductor such as the Ln.sub.2-X Ce.sub.X CuO.sub.4-Y type (wherein Ln is a lanthanide element) are well known.
The hole injection type oxide superconductor needs to be annealed in an oxidizing atmosphere. On the other hand, an electron injection type oxide superconductor needs to be annealed in a reducing atmosphere.
It is noted that the oxide superconductor is applied to an interconnection or wiring layer in an electrical device.
To produce the electrical device a reducing atmosphere is often used. Thus, the electron injection type oxide superconductor which needs annealing in a reducing atmosphere is often used.
A conventional oxide superconductor has been produced by a sputtering method, usually by using a sintered bulk material as a target.
For example, in the case of a Ln.sub.2-X Ce.sub.X CuO.sub.4-Y type oxide superconductor, a sintered bulk material containing the elements Ln (lanthanide), Ce (cerium), Cu (copper), etc., is produced, and sputtering is effected by using the sintered bulk material as a target whereby the superconductor film is formed on a required substrate.
Since a conventional LN.sub.2-X Ce.sub.X CuO.sub.4-Y type oxide superconductor film is produced by a sputtering method, Ce is contained therein in such a manner that it is solid-dissolved in Ln (wherein Ln is a lanthanide element). Therefore uniform addition of Ce or control of the addition thereof has been very difficult.
Furthermore, the film composition was found to contain easily varied phases such as CeO.sub.2, Ln.sub.2 O.sub.7 (wherein Ln is a lanthanide element), etc., other than superconducting Ln.sub.2-X Ce.sub.X CuO.sub.4-Y and the superconducting properties of the obtained superconductor were deteriorated.
Further, in the case of a Bi type oxide superconductor such as (Bi.sub.2-X-Y Pb.sub.X Sb.sub.Y)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 a sintered bulk material made of Bi.sub.2 O.sub.3, PbO, Sb.sub.2 O.sub.3, SrCO.sub.3, CaCO.sub.3 and CuO is used as a target to perform sputtering. In the case of (Bi.sub.2-X-Y Pb.sub.X Sb.sub.Y)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, Sb is contained in such a manner that it is solid dissolved in Bi. Addition of a required or constant amount of Sb is very difficult because the additional amount of Sb is small and Sb is easily aggregated. Thus, the composition of an obtained superconductor is changeable and Sb.sub.2 O.sub.3 or an unidentified phase other than a phase of (Bi.sub.2-X-Y Pb.sub.X Sb.sub.Y)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 is found.